feat: update promo/referral code doc

[dcrawbuck]

Add a note to the 'Referral flows' documentation clarifying custom UI support for paywalls.

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Note

Introduces Mermaid diagrams and refines the referral/promo code documentation.

• New Mermaid client component (src/components/Mermaid.tsx) with themed init, render on mount, and graceful fallback; wired into MDX via src/mdx-components.tsx
• Styling for responsive diagrams added in src/app/global.css
• Dependency mermaid added to package.json; lockfile updated
• Referral guide (content/docs/using-referral-or-promo-codes-with-superwall.mdx) now uses <Mermaid> instead of code fences and includes a note that overlaying custom UI on paywalls is supported only on native iOS/Android SDKs
• next-env.d.ts removed and added to .gitignore

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